1. Field of the Invention
The present invention relates to a closed forging die and a forging method.
2. Description of the Related Art
In manufacturing a product having a boss portion radially provided with shaft portions, such as a trunnion for a constant velocity joint or a cross spider for a universal joint, by closed forging, a closed forging die is used.
As shown in FIG. 8, the closed forging die includes openable dies 1 and 2, and punches 4 and 5 arranged so as to be capable of being driven along center axes of the dies 1 and 2, respectively. That is, by pressurizing a material therein by the punches 4 and 5 while the dies 1 and 2 are in a closed state, a cavity 9 corresponding to configurations of shaft portions 7 and a boss portion 8 of a product 6 is molded. Thus, as shown in FIG. 9A, in a case where, after introducing a billet (material) 10 (refer to FIG. 9A) in the dies 1 and 2, clamping is performed and then the billet 10 is pressurized by the punches 4 and 5, the billet 10 plastically deforms, to thereby configure the product 6 including the boss portion 8 and the shaft portions 7 as shown in FIG. 9B.
That is, as shown in FIG. 9A, in a case where the cylindrical billet 10 having a radius of curvature R2 is introduced in the die and forged, the product 6 having the shaft portions 7 each of whose tip portion 7a has a radius of curvature R2′ larger than the radius of curvature R2 can be molded.
Incidentally, in a case where a sealed state is established in the closed forging die, a processing load drastically increases, which leads to a fear in that the die may be damaged or short-lived. Thus, a related art describes that a length of each shaft molding portion is set larger than a required length of each shaft portion, to thereby provide a clearance to each shaft tip portion (JP 2003-343592 A).
In the related art die formed with the clearance portion in each shaft molding portion, when the billet is pressurized by the punches, the material is extruded to mold the shaft portions. At a tip surface of each shaft portion, a center portion of the extruded material readily flows and a peripheral portion thereof does not readily flow. Thus, as shown in FIG. 4, a shaft portion having a tip surface having a radius of curvature R1′ smaller than a radius of curvature R1 of the tip surface of a normal shaft portion is molded. In this manner, in the conventional die formed with a clearance in each shaft molding portion, “sagging” occurs by which a circumferential surface side is retracted toward a base end portion side of the shaft portion in an axial direction thereof compared to the tip portion of the normal shaft portion.
Thus, in a case of securing the length of the shaft portion accurately molded using the die, the material is additionally required by an amount corresponding to the “sagging.” Incidentally, the forged product molded by using the closed forging die is included in an inner joint member of a constant velocity joint or a universal joint. Thus, in order to make the constant velocity joint or the universal joint employing the product compact and lightweight, it is necessary to machine a tip of the shaft portion to be removed.
In addition, in order to extend a lifetime of the constant velocity joint or the universal joint including the product incorporated therein and to suppress vibration and noise in use, it is necessary that, after increasing strength and hardness of the product by heat treatment, the shaft-portion outer circumferential surface of the product be molded higher in accuracy than that molded by the forging. Thus, it is necessary to finish the product by machining after the heat treatment. The shaft tip may be removed by the machining prior to the heat treatment in order to facilitate the machining after the heat treatment, and a coupling surface of the removed surface and the shaft-portion outer circumferential surface may be used as a reference for phase matching in the case where the shaft-portion outer circumferential surface is subjected to highly-accurate machining. Thus, the coupling portion is required to be formed with high accuracy.